Apple added a new “low power mode” to the iPhone as part of iOS 9. You’ll be prompted to activate it each time your phone reaches 20 percent battery, but you can also enable it before that point to stretch your battery power further.

This changes many of the settings people commonly change when they want to make their iPhones live longer between charges. For whatever reason, low power mode is only available on iPhones, not iPads.

How to Activate (and Deactivate) Low Power Mode

When your iPhone reaches 20 percent battery power left, you’ll see a “Low Power Mode” prompt appear. Your iPhone will inform you what features will be temporarily disabled, and you can choose to “Continue” and enable low power mode or “Cancel” and not enable low power mode. Low power mode can reportedly give you anywhere from one to three hours more time before your iPhone dies.

You can also enable low-power mode whenever you like. For example, let’s say it’s the start of a long day and you know you’ll be away from an outlet for a long while. To do this, open the Settings app, scroll down, and tap the new “Battery” category. Activate the “Low Power Mode” slider to enable it. The battery indicator in the status bar will turn yellow while low power mode is enabled.

Your iPhone will always automatically disable low-power mode when you charge it up to a certain point. Low power mode is always temporary and only lasts until the next proper charge.

What Low Power Mode Does

Low power mode does a number of things to save battery power. It automatically changes some settings to save battery power, disabling automatic fetching of new mail, reducing your screen brightness, and automatically locking the phone and powering off its display more quickly. Apps can detect low power mode is enabled and choose to disable animations and other battery-hungry features, too.

Motion effects and animated wallpapers are also disabled. Background activities and networking are paused to prevent unnecessary power drain in the background. Your iPhone even automatically reduces the performance of its CPU and GPU, which makes it perform a bit slower but saves battery life. Tests have found that this slows iPhones by about 40 percent when low power mode is enabled.

Low power mode is fairly aggressive, which is why it isn’t enabled all the time. It’ll help you squeeze more battery life out of your phone when necessary, but you probably wouldn’t want to use it all the time.

While you can’t permanently enable low power mode, you can permanently change some of the settings low power mode does. You can also use the options on the Battery settings screen to see which apps are using the most battery power and choose to remove them or adjust their settings.

Disable mail fetching: If you have any email accounts configured to “fetch” new mail, your iPhone is automatically checking them at regular intervals and downloading new mail. This forces your iPhone to regularly wake up and do work. Set your mail accounts to “push” new mail to you or just disable this and manually check for new mailto save power. Using manual refresh will prevent you from receiving email notifications, however. It’s a trade-off.

Screen brightness: Enabling auto-brightness will ensure your screen isn’t too bright when it doesn’t need to be, saving battery power. This setting should be on by default — just don’t disable it. You can also swipe up from the bottom of your screen at any time to manually adjust the brightness level. The brighter your display, the faster your battery is draining. This is available under “Display & brightness” in the Settings app.

Auto-lock timeout: To save battery power, you can have your iPhone automatically lock itself and turn off its display after a shorter period of time when you aren’t using it. Open the Settings app and navigate to General > Auto-Lock to find this setting. For example, you could have your display automatically turn off after as little as 30 seconds.

Disable background refresh: You can prevent apps on your iPhone from automatically refreshing in the background while you aren’t using them, too. To do this, open the Settings app and navigate to General > Background App Refresh. You can disable Background App Refresh for every app from here, or just prevent individual apps from refreshing.

There’s no way to permanently throttle down your phone’s CPU or GPU, however. You’ll have to enable low-power mode whenever you want to slow your hardware down to save battery power.

As of iOS 9, your iPhone’s display won’t light up when it receives notifications if it’s placed face-down. Place your iPhone face-down to save some battery power and prevent the display from coming on if you don’t care about receiving notifications at the moment.